Floating of Extended States and Localization Transition in a Weak Magnetic Field

We report results of a numerical study of noninteracting electrons moving in a random potential in two dimensions in the presence of a weak perpendicular magnetic field. We study the topological properties of the electronic eigenstates within a tight binding model. We find that in the weak magnetic field or strong randomness limit extended states float up in energy. Further, the localization length is found to diverge at the insulator phase boundary with the same exponent $\nu$ as that of the isolated lowest Landau band (high magnetic field limit).